how to import namespace

Hello Friends,
Below pgm i tried it  to test onclientclick and onclick event its working fine....But my doubt:
How to import namespace in serverside script i tried <%@ Import Namespace="System.Data.SqlClient" %> below dis line <%@ Page Language="C#" %> but its not working both onclientclick and onclick event....only onclientclick s working at that time...
what to do please help me guys.....

<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<script runat="server">
In this portion i need to import namespace how to do it please help me.....
    protected void btnSubmit_Click(object sender, EventArgs e)
    {
        //Here i done some coding part related to ...